The present invention relates to Identification-Friend-or Foe system, and more particularly to a means of detecting interference or jamming on the reply frequency of an IFF system. In proper utilization of an IFF system, it is important to determine whether an enemy source is causing interference to the reception of replies to interrogations-interference which may prevent friends from being identified as friends or which may cause other kinds of undesirable misoperation. Furthermore, it is important, if interfering signals are present, to know the extent of disruption caused to the Identification process-particularly whether the interfering signals prevent correct processing of replies, or whether replies can be properly processed in spite of the interference.
The Semiautomatic Jam-Accept (SAJAC) Decider described in my U.S. patent application SEMI JAM-ACCEPT (SAJAC) DECIDER FOR MODE-4 OF THE IFF MARK III, U.S. Ser. No. 78,317, filed on Sep. 3, 1970 (presently allowed under secrecy order), was the first Mode 4 Reply Evaluator designed to specifically declare the presence of interfering or jamming signals on the reply frequency when they become serious enough to interfere with the evaluation of replies from friends. The SAJAC device declares jading when extraneous reply-frequency signals are so numerous that the system is jammed. SAJAC does not declare the presence of interfering signals if they are not severe enough to jam the system, even though they may be easily detected as enemy-produced signals.